In optical microscopy, the generation of patterns of light at high spatial and temporal resolution is desirable. A pattern of light refers here to the focusing of light at one or more spots at given positions in 2D and 3D and to the sculpting of the light intensity distribution in an extended pattern around these spots. Indeed, a pattern of light can be used to image the structure and activity of biological samples, such as neuronal dendrites. A pattern of light also enables to modify optically the biological activity or chemical environment of these biological samples. It is therefore desirable to be able to generate arbitrary light patterns at a high speed.
For this, it is known to carry out a three-dimensional scanning of a sample by using galvanometers for in-plane scanning and various mechanical designs for scanning along the optical axis (Z scanning). These designs include displacement of optical elements such as lenses or optically-conjugated mirrors or deformation of liquid lenses.
However, due to the poor spatio-temporal patterning capacity of both galvanometers and mechanical Z-scanning devices, the versatility of this approach is limited. It results in spatially constrained light pattern formation at a relatively low speed.
Spatially extended light patterns are also produced by devices able to spatially shape the light in phase and/or amplitude, including mainly liquid-crystal SLM (Spatial Light Modulators) and DM (Deformable Mirrors). These devices are also limited in their ability to refresh at high rate the modulation pattern, in most of the cases in the range of a few Hz to a few kHz.
In the case of the scanning in 3D of a single spot, it is also known from the article by Reddy et al. entitled “Three-dimensional random access multiphoton microscopy for functional imaging of neural activity” published in Nature neuroscience, 11, 713-720 (2008) to achieve three-dimensional ultra-fast scanning using a system involving four acousto-optic deflectors.
Acousto-optic deflectors (often named after their acronym AOD) are fast pointing devices based on the interaction between an acoustic compression or shear wave propagating in an acousto-optical crystal and an electromagnetic wave. In most cases, the electromagnetic wave has a planar or a spherical wavefront. If a fixed frequency wave is used in the AOD, the resulting diffractive process deflects a fraction of the electromagnetic wave at an angle proportional to the acoustic frequency of the acoustic wave. The use of linearly chirped wave in the AOD, as described in the article by Reddy et al., allow creating a cylindrical lens in the AOD, whose focal length is inversely proportional to the chirp rate.
The system described in this article, which contains 4 AOD, implies strong loss of light power because the diffraction efficiency in an acousto-optic deflector is limited even in its optimal configuration. In addition, the third and the fourth acousto-optic deflectors of the system cannot be used in their optimal configuration (Bragg incidence), which also contributes to increase the loss of light power in the system. Finally, the linear frequency chirps have to be stopped each time they reach the limit of the frequency bandwidth of the AOD, thus limiting the dwell time on the points accessed in 3D and the useful fraction of the duty cycle. Finally, this system is limited to the scanning in 3D of a single spot and cannot be used to create extended light patterns.
It is also known from document DE 10 2013 201 968 a device which has a radiation source generating a pulsed, electro-magnetic radiation. A photodetector is used for temporal detection of radiation pulses. A signal source is coupled with the photodetector for precise trigger of sound waves generation. An acousto-optical deflector is driven by the signal source with a frequency-modulated control signal such that sound waves and radiation are linked in time. In this implementation refractive and/or diffractive beam transformation and/or beam deflection of the radiation is/are performed at the time of passage of the radiation through the deflector.